Add a reference about possibly different frequencies in older Pis.

Addresses #1

README.md

@@ -17,6 +17,12 @@ reception. This magnetical coupling is very low power and only works over a few
 centimeters, but **_before running this program, make sure you follow your
 local laws with regard to restrictions on radio transmissions._**
 
+### Platform
+This runs on a Raspberry Pi. So far, it has been tested on a Pi3 and a
+Pi Zero W. There has been a report of different frequencies generated with
+an older Pi (Bug #1), so until we have a definitive list of available
+clock sources inside these, check out that bug for a workaround.
+
 ### Supported Time Services
 #### DCF77
 The [DCF77] (Germany) signal is a 77.5kHz carrier, that is amplitude modulated

gpio.cc

@@ -161,6 +161,16 @@ double GPIO::StartClock(double requested_freq) {
 
   EnableClockOutput(true);
 
+#if 0
+  // There have been reports of different clock source frequencies. This
+  // helps figuring out which source was picked.
+  fprintf(stderr, "Choose clock %d at %gHz / %.3f = %.3f\n",
+          kClockSources[best_clock_source].src,
+          kClockSources[best_clock_source].frequency,
+          divI + divF/1024.0,
+          kClockSources[best_clock_source].frequency / (divI + divF/1024.0));
+#endif
+
   return kClockSources[best_clock_source].frequency / (divI + divF/1024.0);
 }